GDScript: Fix message when calling non-tool function in tool mode

[dalexeev]

• Closes Improve error message for calling non-tool scripts from tool scripts #36592.
• Alternative to Improve error message when trying to call non-@tool function from @tool script #84045.
• See also Getting False Positive "..Invalid call. Nonexistent function..." when running an EditorPlugin #94499.

Instead of the "Nonexistent function..." error, a more useful message is displayed:

res://addons/BugIsHere/addon_main.gd:15 - Invalid call function 'initialize' in base 'Control (AddonControl)': Attempt to call a method on a placeholder instance. Check if the script has the tool annotation.

[AdriaandeJongh]

Why not something along the lines of "Methods without the @tool annotation can't by called by scripts in tool mode."? "Check if" implies less of a fix.

[akien-mga]

Thanks!

[akien-mga]

No cherry-picking for 4.3.x for now as this caused regressions (fixed with #96173 and #96344, but this calls for some caution/more testing in 4.4).